Apparatus for determining moisture



March 15, 1955 LAMB 2,703,983

APPARATUS FOR DETERMINING MOISTURE Filed April 26, 1951 ANTHONY LAMB IN VEN TOR.

United States Patent APPARATUS FOR DETERMINING MOISTURE Anthony H. Lamb, Hillside, N. J., assignor to Weston Electrical Instrument Corporation, Newark, N. J., a corporation of New Jersey Application April 26, 1951, Serial No. 223,098

6 Claims. (Cl. 73-335) This invention relates to apparatus for determining, electrically, humidity, moisture content and associated water vapor-air relationships and, more particularly, to a novel moisture-sensing element suitable for use with such apparatus.

The determination of the amount of water vapor in gases, materials and substances is becoming of increasing importance. Moisture content is usually expressed as a volume, mass or weight ratio and is given such specific terms as absolute humidity, relative humidity, percent moisture, vapor pressure, dew point, etc. Numerous devices are presently available for establishing or determining moisture relationships but these are, in general, expensive, cumbersome and of limited application. So far as I am aware there exists no simple, inexpensive and reliable device having a broad use in the numerous fiields wherein determinations of moisture content are important. Some specific examples of such fields are, the determination of the relative humidity within a moisture-proof con tainer without destroying the sealed character of the container, the determination of the relative humidity within very small containers such as an electrical instrument having a casing of /2" diameter and 1" length, the determination of the water vapor content of a single cigarette, the determination of the water content of food in either loose or packaged form.

The present invention contemplates the provision of a moisture-sensing element made of inert, hygroscopic material which responds to water vapor changes by changes in its ohmic resistance value in a repeatable and reversible manner and which is simple, inexpensive, and reliable. Such element can be made in any desired size as, for example, for insertion into a cigarette, and can be calibrated with an indicating instrument to provide a direct reading of the water vapor content of any given gas, material, or substance. While my novel moisturesensing device is adapted for use in many fields, the present description will be restricted to its use in the determination of the relative humidity of the air contained within a hermetically sealed, electrical instrument and the determination of the water vapor content of matter sealed within a moisture-proof package. Such restricted description is believed suflicient to convey a proper understanding of the invention and those skilled in this art will have no difiiculty in applying the sensing element, and the associated apparatus, to other specific uses.

An object of this invention is the provision of a simple, inexpensive moisture-sensing element whose electrical characteristics change with the amount of water vapor in the medium surrounding the element.

An object of this invention is the provision of a moisturesensing element whose electrical conductance changes with the amount of absorbed and/ or adsorbed moisture in a repeatable and reversible manner.

An object of this invention is the provision of an inert, hygroscopic moisture-sensing element which is adapted for use with a suitable electrical network for the determination of the water vapor content of the medium surrounding the element.

An object of this invention is the provision of apparatus for the determination of the water vapor content of a medium contained within a moisture-proof container without destroying the sealed character of the container.

An object of this invention is the provision of moisturedetermining apparatus comprising a moisture-sensing element whose electrical characteristics change with the "ice moisture content of the surrounding medium, an electrical network including an indicating instrument having a scale calibrated in terms related to moisture content, and leads by which the element can be connected into the electrical network.

An object of this invention is the provision of a hermetically sealed electrical indicating instrument and including self-contained means for determining the relative humidity of the contained air.

These and other objects and advantages will become ap parent from the following description when taken with the accompanying drawings illustrating several embodiments of the invention. It will be understood the drawings are for purposes of illustration and are not to be construed as defining the scope or limits of the invention, reference being had for the latter purpose to the appended claims.

In the drawings wherein like reference characters identify like parts in the several views:

Figure 1 is a top, plane view of an electrical instru mental representative of the type of device to which the invention is particularly well suited;

Figure 2 is a side elevation of such instrument with parts broken away for purposes of description;

Figure 3 is an isometric view of a moisture sensing element made in accordance with this invention;

Figure 4 is a fragmentary view showing the moisturesensing element secured to two terminals of an electrical instrument and including a schematic diagram of a circuit suitable for determining the moisture content of the contained atmosphere;

Figure 5 is a similar to Figure 4 but showing an arrangement wherein the instrument mehcanism is employed to indicate the moisture content of the contained air;

Figure 6 illustrates an arrangement wherein the moisture-sensing element it imbedded in material contained within a moisture-tight package and such element is employed as a component of an electrical network for the determination of the moisture content of the material.

Figures 1 and 2 illustrate a typical electrical instrument of the hermetically sealed class. The instrument is of the core magnet type comprising a substantially cylindrical permanent magnet 10 supported in spaced relation to a rectangular, soft-iron yoke 11. The movable coil 12 is pivotally mounted for rotation in the magnetic flux gaps formed between the magnet and the yoke, and has secured thereto a pointer 13 that cooperates with a calibrated scale 14 carried on the scale plate 15. Current is conducted to the movable coil by a pair of conventional spiral springs, one such spring 16 being shown in Figure 2. The movable coil is secured to axial staffs that are disposed on opposite sides of the coil and which rotate within jewel bearings, the upper staff 17 and bearing 18 being visible in Figure 2. Rotatably secured to the upper portion of the yoke 11 is an abutment 19 one end of which is secured to an end of the upper spring 16. The opposite end of the abutment includes an elongated slot 20 which accommodates the pin 21 of the zero adjuster mechanism, which mechanism is of the type disclosed in my United States Patent No. 2,513,184; issued June 27, 1950. Such zero adjuster mechanism permits adjustment of the pointer position, externally of the instrument casing, by rotation of the slotted number 22, without destroying the hermetically sealed character of the instrument casing.

The instrument mechanism is contained within a casing 25 having a transparent cover 26 at one end. Such cover has a metallized edge and is solder-sealed to the casing to provide a moisture-tight joint, as is well known in the art. The instrument terminals are also of the solder sealed type, one such terminal being shown in Figure 2. These terminals may comprise a metal rod 27 imbedded in a ceramic cylinder 28, the latter being provided with a metalized section whereby the metal flanges 29 can be soldered to the cylinder and to the base 30 of the instrument casing. Those skilled in this art will know that each of the instrument springs is connected to such terminals and that the instrument is connected to an external electrical circuit by means of leads secured to the external portion of the terminals as by means of the nuts 32.

The above described instrument represents one construction commonly referred to as a hermetically sealed instrument, which construction is designed to protect the delicate instrument mechanism from damage by moisture. In such hermetically sealed instrument it is, of course, essential that the air contained within the casing be devoid of moisture. This can be accomplished in several ways as by heat-drying the air, placing a hygroscopic material in the casing, replacing the contained air with an inert gas, etc., all just prior to the final sealing of the instrument casing in the manufacturing process. It is desirable to provide some means whereby the user of the instrument can check the quality of the contained air without destroying the sealed character of the instrument. Such check serves not only as a warning in the event the casing develops an air leak but also to assure the user the instrument is in proper condition for use, particularly in areas having high humidity. At present, so far as I am aware, there is no simple, convenient means for such purpose.

Reference is now made to Figure 3 which is an isometric view illustrating one form of my novel moisturesensing element. The element comprises a rod 35 of inert, hygroscopic material capable of absorbing and/or adsorbing moisture from the surrounding medium and whose electrical characteristics vary with the amount of its contained water vapor in a reversible and repeatable manner. Such material should, preferably, be of a cellulosic character such as balsa wood, fir wood, compressed cotton, etc. I have found balsa wood highly satisfactory for the purpose as it is inexpensive, easy to shape to a desired form and, importantly, is highly responsive to small moisture changes in a repeatable and reversible manner. Such moisture-sensing element can be connected into a suitable electrical network by means of the wire leads 36 attached to spaced points of the material. It is here pointed out that such leads are fastened to the material by passing an end through the material, entwining the wire around the circumference under tension so that such encircling turns are partially imbedded in the material, and joining the protruding end of the wire to the last turn thereof, substantially as shown. By so attaching the lead wires the electrical characteristics of the material, between the two leads, will vary in accordance with the moisture absorbed or adsorbed at the surface as well as by the moisture absorbed by the inner portion of the material by reason of its longitudinal, cellular nature. Since the electrical conductance through the material, as measured across the leads 36 changes with the water vapor content of the material, such characteristic can be employed to provide a simple direct-reading apparatus for indicating the water vapor content of the atmosphere surrounding the sensing element as a whole. It will be apparent the size and shape of the sensing element, and the spacing of the points of attachment of the leads, can be varied to meet the requirements of specific applications having in mind the nature of the atmosphere or substance surrounding the element and the characteristics of the electrical circuit to be associated therewith. The calibration of a moisture-sensing element for incorporation in a given electrical network and for a specific moisture-determining use can be established in the laboratory.

Reference is now made to Figure 4 which is a fragmentary representation illustrating the use of my sensing element in a hermetically sealed instrument whereby the relative humidity, or moisture content of the contained air, can be determined externally of the instrument casing. Here I show three terminals 28, 28' and 28", passing through the base 30 of a hermetically sealed instrument. The instrument construction, for example, can be of the type shown in Figures 1 and 2 although Figure 4 shows the movable coil 12 and the pointer 13 schematically. The movable coil is connected to the terminals 28, 28" whereby the coil can be connected to an external electrical circuit by the leads 37, 38 attached to the external portion of said terminals. A moisture-sensing element 35 is disposed within the instrument casing with its leads 36 connected to the terminals 28, 28 whereby the element can be connected into an external network comprising a source of fixed voltage 40 and an indicating instrument 41, as shown. The series circuit comprising the voltage source 40, instrument 41 and the sensing element 35 consists of components having predetermined constants, that is to say, the magnitude of the voltage of the source 40 is such that the current flow through the sensing element is a predetermined value at a given relative humidity of the air surrounding the element. The instrument 41 can then be provided with a scale calibrated directly in terms of relative humidity. Thus, if the source 40 has an electrical potential of, say, 1 volts and the moisture-sensing element is thoroughly dry (in which case the resistance across the leads 36 is a maximum) a minimum current flows through the instrument 41. The position of the pointer of such instrument, under such condition, will correspond to the zero (0) relative humidity graduation on the scale. As the relative humidity of the air surrounding the sensing element increases, the resistance of the element decreases resulting in an increasing current flow through the instrument 41. The top mark of the instrument can, therefore, be established at any specific desired relative humidity value de pending upon the pointer-deflection range best suited for the particular application. As an example, a scale range of 050 per cent relative humidity is ample for an application of the type under discussion while a range of 3()l00 per cent relative humidity may be more suitable for applications wherein the encountered humidity never falls below 30%.

It will be apparent the incorporation of the moisturesensing element within the hermetically sealed instrument does not in any way interfere with the normal use of the instrument. The sensing element is permanently enclosed within the instrument casing and, since its conductance at any instant is related to the humidity of the contained air, the user can at any time determine the actual humidity within the instrument by connecting the proper voltage source 40 and a properly calibrated instrument 41 to the terminals 28, 28'. From a practical standpoint, I have made moisture-sensing elements of this type having an overall size of inch diameter and inch length, which elements are suitable for incorporation in very small instruments, containers and spaces.

Figure 5, which is similar to Figure 4, illustrates an arrangement wherein the normal instrument mechanism can be employed for determining the moisture-content of the contained air thus obviating the need for a separate instrument for this purpose. As in the Figure 4 arrangement the instrument movable coil 12 is connected to the instrument terminals 28, 28". The pointer 13 cooperates with a scale 45 which may be calibrated in terms of volts, amperes, ohms, etc. depending upon the use for which the particular instrument is intended. The moisture-sensing element is connected between the movable coil and the third terminal 28, as shown. If, now, the source 40 of constant voltage is connected to the terminals 28, 28", the deflection of the movable coil 12, and its pointer 13, will depend upon the resistance of the circuit comprising the movable coil and the sensing element. As explained hereinabove, the resistance of the sensing element depends upon the moisture content of the surrounding air. With proper consideration given to the resistance of the movable coil, the resistance range of the sensing element from the dry to the wet state, and the actual voltage of the source 40, a second scale 46 can be provided, such scale being calibrated directly in terms of relative humidity. Since the voltage source may comprise a conventional dry battery the ease with which the user can check the internal moisture content of the instrument is apparent. There are now available small batteries of so-called 5 year life type. In instruments of suitable size such battery can be disposed within the sealed casing and the moisture-checking circuit put into operation by closure of a moisture-proof switch carried by the casing. The device, therefore, can be made entirely self-contained. In fact, such self-contained construction is highly desirable in connection with submersion-proof instruments, apparatus, test sets, etc. wherein the exterior casing is of the pressure-sealed, or gasketed type. In the latter case, the casing can be opened to afford access to the components, and re-sealed. Replacement of the battery employed in the moisture-checking network presents no problem.

Figure 6 illustrates the use of my moisture-sensing element in connection with material contained within a moisture-tight package. Here the sensing element 35 is surrounded by the contained material 50 with the element leads 36 passing outwardly of the package 51. These leads are of very fine wire in order not to interfere with the sealed character of the package which, usually, is wrapped with one or more layers of parafiin, or plastic coated, paper. The thin lead wires pass between the paper layers. Alternatively, the leads can be brought out through individual spaced holes in the package which openings are subsequently sealed with wax. Obviously, in cases where the contained material need not be sealed within a moisture-proof container, such precautions need not be observed. In any event, the leads extending from the package serve as a means for connecting the moisture-sensing element to suitable apparatus whereby the moisture content of the material can be established conveniently and reliably. Such apparatus may comprise a battery 52, a switch 53, a current-limiting resistor 54 and an indicating instrument 55. When the switch 53 is closed, closure of the double-pole, double-throw switch 56 in the upper, or Test, position, places the standard resistor 57 into the circuit. Under this condition the adjustable resistor 58 is adjusted until the pointer of the instrument indicates a designated mark on the associated scale thereby setting the entire apparatus in proper condition for moisture-reading purposes in relation to the specific sensing element. The double-throw switch 56 is now thrown to the lower, or Check position, whereby the sensing element is substituted into the electrical network in place of the standard resistor 57. The indication of the instrument 55 then provides a direct reading of per cent moisture content of the material 50. It may here be stated that the leads 60 by which the sensing element is connected to the apparatus may be provided with spring clips for quick attachment to the leads 36 of the element.

My novel moisture-sensing element makes it feasible and practical to incorporate such element in a casing, package or other container in a more or less permanent manner. Its small size, low cost, quick, reliable and convenient operation increases significantly the fields of use wherein moisture-determinations are necessary or desirable and, more importantly, provides a means for such determinations with respect to applications for which satisfactory apparatus or method is now lacking. While I have restricted the description to several specific applications, those skilled in this art will have no difficulty in applying the invention to other uses. The use of the Figure 6 arrangement for determining the relative humidity of free air, or the water vapor content of loose matter such as tobacco, sugar, cotton, etc., is quite apparent. In each specific case the particular sensing element is designed for the specific use, the electrical network is designed for operation with such element and the indicating instrument scale is calibrated in appropriate factors such as per cent humidity, water content/pound or /volume, etc. Alternatively, the scale can be calibrated in terms of ohms or arbitrary units and the instrument reading converted to the desired moisture factors by reference to a suitable chart or tabulations.

Having now described my invention in detail in accordance with the patent statutes, what I desire to secure by Letters Patent is set forth in the following claims.

I claim:

1. An electrical instrument of the type including a movable coil carrying a pointer cooperating with a scale, and terminals extending through the case for connecting the movable coil to an external electrical circuit, and including a moisture-sensing element disposed within the instrument casing said element having an electrical conductance that varies with its water content, a third terminal extending through the instrument casing, and a pair of leads passing through the element at spaced points, one of said leads being connected to one side of the movable coil and the other lead being connected to the said third terminal.

2. A sealed electrical instrument comprising a casing and provided with a transparent window, three terminals extending through the casing, a movable coil disposed within the casing and connected across two of said terminals, a pointer attached to the movable coil, a moisture-sensing element disposed within the casing said element having an electrical conductance that varies with its water content, a first lead passing through the element and connected to one side of the movable coil, a second lead passing through the element and connected to the third terminal, and a scale cooperating with the pointer, said scale being calibrated in factors related to the water content of said element.

3. The invention as recited in claim 2, wherein the moisture-sensing element is made of a cellulosic material and the said scale is calibrated in terms of relative hu midity.

4. An electrical instrument of the type comprising a movable coil carrying a pointer cooperating with a scale and terminals extending through the instrument case for connecting the movable coil to an external circuit, and including a moisture-sensing element having a conductance that varies with its moisture content, a pair of leads attached to the element at spaced points each lead being entwined under tension peripherally about the element and including a portion that is pierced through the element in a plane substantially normal to the element axis, a substantially constant voltage source, and means operable to connect the said element and movable coil in series across the voltage source.

5. The invention as recited in claim 4, wherein the said scale is calibrated in terms of relative humidity.

6. An electrical instrument of the type comprising a movable coil carrying a pointer cooperating with a scale and terminals extending outwardly of the instrument case for connecting the movable coil to an external circuit, and including relative humidity values on the scale, a moisture-sensing element having a conductance that varies with its moisture content, a pair of leads attached to said element at spaced points, a substantially constant voltage source and means operable to connect the said element and movable coil in series across the voltage source.

References Cited in the file of this patent UNITED STATES PATENTS 1,749,826 Lubach Mar. 11, 1930 1,843,234 Karnes et al. Feb. 2, 1932 2,047,638 Kott July 14, 1936 2,064,651 Fiene Dec. 15, 1936 2,234,858 Brown et al Mar. 11, 1941 2,371,299 Johnson Mar. 13, 1945 2,425,250 Lamb Aug. 5, 1947 2,476,909 Ratz July 19, 1949 2,512,234 Lamb June 20, 1950 FOREIGN PATENTS 314,675 Great Britain July 4, 1929 

